Method and device for digestion of sludge

ABSTRACT

Method and device for digestion of sludge, sludge being transferred stepwise to closed containers (10A, 10B) and gas generated during the digestion process being transferred from said containers (10A, 10B). The sludge is transferred to a first closed space (11A) in a first container (10A) for a first digestion period, and is then after a predetermined period transferred to a second closed space (11B ) in a second container (10B) for a second digestion period. During said first digestion period as well as during said second digestion period the sludge is repeatedly put into movement by tipping said containers (10A, 10B), the flow direction of said sludge being changed to redirect surface material of the sludge to bottom material by the movement of the sludge. Each of said containers (10A, 10B) is provided with an inlet (12) for sludge and a first outlet (13) for gas generated during the digestion period. Said first closed space (11A) and said second closed space (11B) are connected to each other through a valve (16), and internally they are provided with flow redirectors (22) so as to improve the homogenisation of the Said containers (10A, 10B) are designed to be tipped between two positions.

TECHNICAL FIELD

The invention relates to a method and a device for digestion of sludgeaccording to claim 1.

BACKGROUND ART

In many countries there is a short supply of water and it is costly toconstruct sewers of an appropriate slope to use water as a transportingmedium. By choosing to have an early separation the transport isfacilitated and the need of water is reduced. A lower amount of waterwill also facilitate an efficient treatment of sludge. By constructing aseparating device as close as possible to the origin of the waste waterthe problems discussed above can be reduced and the quality of thesludge improved.

Such an early separation is described in SE 9000921 according to which asewer system having a plurality of separating tanks is used.

To use digestion of sludge for reducing biological material and odor inthe treatment of sludge is well known from a plurality of patentdocuments. Such use is described e.g. in SE 7801404 and SE 7909320.

Also DE-A1-3 230 010 disclosed a device for digestion of sludge. A tankdivided into four chambers is used for the digestion. The tank can betipped between two positions. The digestion process takes part in abottom layer and some sludge will float up to the surface where it isdigested only slowly or not digested at all. Biogas which is formedduring the process presses liquid from one chamber into the second, andthe tank is tipped from slightly tipping in one direction to slightlytipping in another direction. Valves within the tank are used for thepassage of gas in an appropriate way. According to the patent documentthe steering of sludge should be slow and without turbulence, and aplurality of rotatable flaps and similar devices are used. Thereliability in operation of the system described in DE-A1-3 230 010tends to be low as a result of using movable parts and valves within thetank.

Problems that are not considered completely presently when devices forthe digestion of sludge are used are e.g. odor and extended requireddigestion periods resulting in high demands on large sludge drying bedsand on a high quality of the sludge and also to the concentration tolarge plants.

An object of the present invention is to essentially solve the problemsdiscussed above such as large volumes, odor, low quality of sludge andspecially made material. Another object is to accomplish a reliabledevice.

The object has been obtained by including in the invention the featuresof claim 1.

SUMMARY OF THE INVENTION

The invention relates to a device for digestion of sludge in sewingsystems comprising a dewatering chamber, a separate predigestorconnected to a separate post digestor through a connecting line havingan automatically controlled valve, said post digestor in turn beingconnected to a sludge receiver which is fed by sludge as well as mulch.The digested sludge material is automatically laid out in heaps for amaturing stage and can be mixed with garden waste to form a finalproduct to be reused as a fertiliser. The process is anaerobic and gasis formed, which can be used for the heating of the sludge to increasethe digestion efficiency and to decrease the digestion period. Thesewing water comprises inter alia in solution the fertiiising compoundsphosphorous and nitrogen and sometimes also added grinded sludgematerial from kitchen mills. Garden waste can also be added to thesludge to increase the amount of gas. Particles and sludge are separatedat an early stage close to the source in a special separator and aretransported by a pressure or vacuum system in thin tubes to a sludgetreatment plant without generating any odor as a sanitary nuisance.

In the treatment plant the separated sludge first enters a second closeddewatering chamber, an inlet thereof being provided at a high positionto accomplish a sedimentation and further thickening of the sludge.Excess water is led to a filtration tank to be treated. The thickenedsludge is led automatically without any supply of air to a firstseparate digestor which suitably is circular and has a volume which isadapted to the expected load. Inside said digestor is provided with adevice which facilitates homogenisation of the sludge during fasttipping movements of the digestor. The tipping takes place withincertain time intervals and at an appropriate speed. The control of thetipping movement can be automatized. The digestor is externally providedwith a heating device connected to a digestor gas combustion/biogasplant. A temperature increase of the sludge will accelerate thedigestion and accordingly decreases the duration of stay within thedigesters.

When a selected time period has expired the sludge is transferredautomatically from a step one digestor to a step two digestor for acorresponding digestion period. Both digesters are similar and formtogether a module unit for digestion. If there are larger amounts ofsludge further modules can be connected in parallel to form a closedplant.

The anaerobic microbial population is larger in the step two digestor,because this digestor is not exposed to oxygen containing water.

It could therefore be appropriate to retransfer some part of the sludgeof the step two digestor to the step one chamber before refilling thestep one digestor so as to increase the speed of degradation and theproduction of gas. Such a retransfer can take place through a furthercontainer. Instead it would be possible to transfer only a part of thesludge from the step one digestor to the step two digestor.

The digestion time in one module is approximately 14 days, whereafterthe sludge can be discharged into a screw milling the sludge togetherwith mulch to a thick consistency adapted to be put out automatically inheaps on a sludge drying bed. After maturing of the sludge the materialis suitable to be spread out on the fields or the soil.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described by means of practical embodiments,reference being made to the accompanying drawings, in which

FIG. 1 shows a device according to the invention,

FIG. 2 is a basic drawing showing a device according to the inventionfor use within one field of application,

FIG. 3 shows an alternative embodiment of a device according to theinvention, and

FIG. 4 is a side elevational view of a part of the device according toFIG. 2.

DESCRIPTION

FIG. 1 shows that the device according to the invention comprises aclosed free-standing dewatering chamber 29 and two separate digestors orcontainers 10A and 108 in series. Said dewatering chamber 29 is provided10 with an inlet 27, a valve28 and an outlet valve26 having an outlet.Through a coupling 12 said dewatering chamber is connected to saiddigestor 10A. The function of a presedimentation chamber is to pre-treatthe sludge to obtain an appropriate water content. If the sludge is notthickened enough the digestion will be less efficient. An appropriateconcentration is at least 4-5 percent dry matter of the sludge enteringthe digestors.

In each of the digesters 10A and 10B a liquid space 11A and 11B,respectively, is enclosed. Said digesters 10A and 10B are mounted on acradle 18, resting on a support 20 having a centered arrangement 33which allow a varying slope against the horizontal plane. The fasttipping movements of the cradle is in the shown embodiment accomplishedby a lifting device 34. The fast tipping movement makes the liquidwithin the digesters move heavily which results in good homogenisationof the sludge. In a preferred embodiment said digesters 10A and 10B arecircular cylindrical.

Said digestor 10A is provided with a coupling 12 and an outlet 15 havingan automatic valve 16 through which the sludge is passed on to thechamber 10B in step two for a second period of digestion. Said chambers10A and 10B are internally provided with flow redirectors 22facilitating a homogenisation of the sludge during the fast tippingmovements. Said flow redirectors 22 are provided within said chambers10A and 10B radially projecting from a wall. In a simple and preferredembodiment said flow redirectors 22 comprise discs arranged centrally-inaxial direction in the chambers and radially projecting approximatelyone third of the diameter of the chambers from that side of the chambersforming the upper side when the chambers are in a horzontal position. Asa result of this arrangement and design there will be strong redirectionof the direction of the flow and following from that a homogenisation ofthe sludge during the fast tipping movements of the chambers. Preferablysaid flow directors 22 are arranged to accomplish a similar redirectionfunction in both directions of tippings. Therefore, said flow directors22 are provided approximately in a central position in the axialdirection of both chambers 10A and 10B.

Externally both chambers are provided with heating devices 14. In apreferred embodiment a heating device 14 is connected to a biogascombustion unit or a heating element 23. During the digestion digestorgas is generated, and the digestor gas is taken care of according to theshown embodiment through a connecting line 13 on top of the tank. Thegas line opens in a gas-holder 24.

When a suitable volume of sludge provided through said inlet 12 has beenreached in chamber 10A a valve is closed in the inlet 12. After a fewdays the excess liquid which has been formed in the chamber 11 istransferred by means of gravity discharge to a liquid container 38provided with a valve/connector 37, said container 38 being providedwith a prefilter 35 made of turf and a postfilter 36 made of turf forthe reduction of phosphorous and nitrogen and smaller solid particlesaccompanying the liquid. After passage of said turf filters the liquidis passed on to infiltration or to a normal sewage treatment plant.

The thickened sludge is now digested during a first period of time insaid chamber 10A. During that time there is also provided ahomogenisation of the sludge through fast tipping movements by some timeintervals during which said flow redirectors 22 in the chamber willcreate wave motions that will result in a homogenisation of the sludge.The gas generated in the chamber 10A is led automatically through aconnection pipe to said gas-holder 24 where the gas is partly purifiedby passing through a liquid bath while the purified gas is led to saidheating device 23. Heated water is produced in said heating device 23and is used to increase the temperature in the chamber 10A toapproximately 35° C. to accelerate the digestion process. After adigestion period adapted to the supply of sludge through said inlet 12and in said digestor 10A a lock in the outlet 15 and said valve 16 isopened and the sludge in the stage one chamber 10A is automaticallytransferred to step two chamber 10B in which the same process will takeplace as in 10A. The digestion in the chambers 10A, 10B is at least fivedays. Preferably the digestion period is at least seven days. Duringthis time period the tipping movements are repeated at suitableintervals, preferably with a few hours repetition time. Sludge can befilled alternatively in module units 1 and 2.

FIG. 2 shows what happens after the second digestion procedure inchamber 10B. The sludge is transferred through an outlet valve 17 to ascrew device 30 where the digested sludge is mixed with mulch in afeeding device 19 and with inter alia garden waste in a second feedingunit and then put out in heaps 31 in a bed 32 arranged for this purpose.Then there is a maturing stage before the compost material is collectedfor manuring or to be put into bags.

FIG. 3 shows an alternative embodiment of one of the main components ofthe invention. In contrast to the embodiment according to FIG. 1 thecradle 18 with the containers 10A and 10B is pivotable within a morelimited angle area limited by stops 40. Said stops are provided on thesupport 20. The inclination of the containers 10A and 10B allowed inthis embodiment is sufficient to steer the sludge as desired and also tohave a gravitational flow from the container 10A to the container 10Bwhen the valve 16 is opened.

FIG. 4 shows in an elevational view how biological material, such asmulch or garden waste from a feeding device 19, is mixed with digestedsludge. The mixture is then transferred through a screw device 30 toheaps 31 in a bed 32.

The highly compact embodiment of the device according to the inventionwill considerably facilitate within the area of application describedabove, but also within other areas where a module extension is possibleand an adaptation can be made to incoming volumes of sludge in relationto the chamber 10A, viz. to the population data. The almost odour-freemethod of digestion will allow a location of the plant close to built-upareas, however, under a roof.

We claim:
 1. A method for digestion of sludge, according to which sludgeis transferred to a closed container and gas generated during thedigestion process is transferred from said container, said methodcomprising the steps of:transferring the sludge to a first closed spacewithin said container for a first period of digestion, transferring saidsludge after a predetermined time period to a second closed space in asecond container for a second period of digestion, forcing the sludge tomove back and forth while redirecting the flow of the sludge during saidfirst period of digestion as well as during said second period ofdigestion by repeatedly tipping said container back and forth by meansof a lifting device operatively connected to said containers, andredirecting wave motions generated by said tipping back and forth byflow redirectors arranged within said containers, thereby providing agood homogenization of the sludge.
 2. A method according to claim 1wherein the transfer of sludge between said containers is made without asupply of air.
 3. A method according to claim 1 wherein the transfer ofsludge between said containers is made by a gravitational flow.
 4. Amethod according to claim 1 wherein said first period of digestion isequally long as said second period of digestion.
 5. A method accordingto claim 1 wherein said first period of digestion is at least five days.6. A method according to claim 1 wherein said first period of digestionis at least seven days.
 7. A method according to claim 1 furthercomprising the step of heating the sludge in at least one of saidcontainers so as to accelerate the process of digestion.
 8. A methodaccording to claim 1 further comprising the step of heating the sludgewith gas generated by the digestion.
 9. A method according to claim 1,further comprising the step of mixing digested sludge with biologicalmaterial in a maturing stage.
 10. A device for digestion of sludge, saiddevice comprising:a first container defining a first closed space forreceiving the sludge for a first digestion period, a second containerdefining a second closed space for receiving the sludge for a secondperiod of digestion, each of said and second containers having an inletfor sludge and a first outlet for gas generated during said digestionperiods, said first closed space and second closed space connected toeach other through a valve, the outside of said containers connected toa mechanical lifting device so as to be forced to tip back and forthbetween two separate end positions, said first container and said secondcontainer internally provided with flow redirectors, so as to redirectthe flow direction of the sludge and to improve a homogenization of thesludge, and said valve provided in a connection between said firstcontainer and said second container, said valve opening after apredetermined period of digestion when said first closed space isdisposed at a higher level than said second closed space.
 11. A deviceaccording to claim 10, wherein at least one of said containers isprovided with a heating device for heating the sludge in said container.12. A device according to claim 11 further comprising heating meansconnected to said heating device, wherein said outlet for gas of atleast one of said containers is connected to said heating means for atransfer of digestor gas.
 13. A device according to claim 10, whereinsaid valve provides a transfer of sludge from said first space to saidsecond space through a gravitational flow without supply of air.
 14. Adevice according to claims 10, wherein said containers are mounted on atipping support.
 15. A device according to claim 14, wherein saidtipping support provides tipping of said containers from a horizontalposition to positions of at least +30° from said horizontal position andthen back again.
 16. A device according to claim 10, wherein said flowredirectors comprise a flat disc projecting from an internal wall ofsaid containers for steering of the sludge during the tipping movement.17. A device according to claim 10, wherein said flow redirectorscomprise a disc having a taper wave shape in cross-section projectingfrom an internal wall of said containers.
 18. A device according toclaim 10, wherein said containers are circular cylindrical.
 19. A deviceaccording to claim 10, further comprising a dewatering chamber forthickening of the sludge to a suitable concentration of dry matter, saiddewatering chamber is connectable to said first container for a transferof thickened sludge.